Medium and low-alloy tool steels, known as mold steels, are used to make molds for injection plastic molding and zinc die casting. Such steels are usually supplied prehardened. By prehardened I mean that the steel is hardened before the machining of the die cavity takes place, so that the mold can be placed directly into service after the cavity is machined.
Such prehardened mold steels are usually supplied as billets, plates, or bars, and must have a combination of deep hardenability and high machinability in the hardened condition.
Two currently supplied steels for these applications are AISI P20 and another prior art steel referred to herein as "Mold Steel". The aim analyses of these steels are listed in Table I.
Table I ______________________________________ Type Steel Mold Steel P-20 ______________________________________ C .47 .30 .55 .37 Mn .75 .70 1.25 .90 P .025 Maximum .025 Maximum S .06 .025 Maximum .13 Si .20 .35 .35 .55 Cr .80 1.55 1.25 1.75 Mo .15 .35 .25 .42 B .0005 Minimum None ______________________________________
Because of its carbon content, Mold Steel is not water quenchable; it must be oil quenched, which means that this steel cannot be supplied prehardened from a modern plate mill which employs water quenching. Quenching must be performed in an oil bath which is a separate operation and therefore an added expense. Also Mold Steel, while it is resulphurized, would be a better product if it had higher machinability in the hardened condition. The P-20 steel has the advantage that it is hardenable by water quenching. However, because of its low sulphur content, prehardened P-20 exhibits a machinability that is even lower than Mold Steel. Also, P-20 is relatively expensive to produce because of the high chromium and molybdenum levels necessary to achieve the desired hardenability.
It is well known that boron can be used to replace some of the chromium and molybdenum for hardening, provided that elements such as aluminum and titanium are present to protect the boron. However, it is widely believed that deoxidation with aluminum decreases the machinability of a resulfurized steel because hard aluminum oxide particles are formed and these particles accelerate tool wear. Also, the aluminum changes the sulfide from globular to a stringer-type sulfide concentrated at grain boundaries, and this change also decreases machinability. The negative effects of aluminum on machinability are documented in such publications as "The Making, Shaping and Treating of Steel", Ninth Edition, Page 1286, and U.S. Pat. Nos. 3,424,576 and 3,600,158 to Fogleman et. al. and Molnar et. al., respectively.
There is a need for a steel which is deeply hardenable by water quenching, which is inexpensive to produce because it has a minimum of alloying agents for hardenability, and which is highly machinable in the hardened condition.